1. Field of the Invention
Aspects of the present invention relate to a method of recording holographic information, and more particularly, to a method of recording holographic information that ensures uniform recording quality of a mark and inhibits the occurrence of aberrations in recording/reproduction spots during recording of data on a plurality of recording layers to improve signal quality.
2. Description of the Related Art
In recent years, data storage technology using holograms has attracted considerable attention. A method of storing data using holograms includes storing data in the shape of optical interference fringes in inorganic crystals or in polymer materials that are sensitive to light. An optical interference fringe is formed by two laser beams with coherency. That is, a reference beam and a signal beam travelling along different paths interfere with each other to form an interference fringe that causes chemical or physical changes to a photosensitive storage medium, thus recording data in the photosensitive storage medium. In order to reproduce data from the recorded interference pattern, a reference beam similar to the reference beam used for recording the data is irradiated to the interference pattern recorded in the photosensitive storage medium. As a result, diffraction occurs due to the interference pattern, so that the signal beam is restored to reproduce the data.
The above-described hologram data storage technology includes a volume holography method in which data is recorded and reproduced in page units using volume holography and a micro-holography method in which data is recorded and reproduced bit by bit using micro-holography. In the volume holography method, a vast amount of data may be processed at the same time, but it is difficult to make data storage devices based on this method commercially available since an optical system based on the volume holography method requires very fine adjustments.
In the micro-holography method, two condensed beams are directed to interfere with each other at a focus to form a fine interference fringe. Data storage is accomplished by moving the fine interference fringe on a plane of a data storage medium so that a large amount of data is recorded, thereby forming a recording layer. A plurality of such recording layers may be formed in a depth-wise direction of the data storage medium by superposition, thereby three-dimensionally recording data on the data storage medium.
In order to increase recording capacity in the micro-holography method, a plurality of recording layers may be formed or data may be recorded by overlapping a plurality of wavelengths using wavelength-selective reflection characteristics of a hologram.
However, during a recording process, the sensitivity of a photosensitive data storage medium in which a hologram is recorded (hereinafter, referred to as “holographic data storage medium”) gradually decreases. Specifically, the sensitivity of the holographic data storage medium to incident energy is reduced during the recording process so that even if data is recorded with the same energy, the diffraction efficiency of the hologram may be less than at the beginning. Accordingly, when data is recorded with the same energy, a portion where at least two holographs overlap one another has reduced diffraction efficiency. Therefore, a portion where a previously recorded mark overlaps an additionally recorded mark may have a varied reflection rate in a multi-wavelength recording mode.
Also, when data is recorded to form a plurality of recording layers, a plurality of hologram marks are present in a space through which recording/reproduction beams travel. A portion of the holographic data storage medium where the hologram mark is formed has a different refractive index from nearby portions. Since the phase of light directed at a particular position of the holographic data storage medium including a hologram mark is altered due to the difference in the refractive index of the hologram mark compared to surrounding regions, a wave front of the light is varied by as much as a non-uniformity in a refractive index distribution. Accordingly, when data is recorded on or reproduced from a recording layer disposed far from the surface of the holographic data storage medium, a wave front of light traveling through an already recorded hologram mark is distorted, and the wave front distortion (or wave front aberration) affects the recording beams. An increase in wave front aberrations may lead to deformation of optical spots that converge into a focus, thus increasing the size of the optical spots. The increase in the size of the optical spots may result in deterioration of formation of a recording mark in a recording mode and quality degradation of a reproduction signal in a reproduction mode.